There is provided an isomerization process using a catalyst comprising a hydrogenation/dehydrogenation component, such as a noble metal, and an acidic solid component comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal. This catalyst is used to isomerize, for example, C.sub.4 to C.sub.8 paraffins. The feed to this paraffin isomerization reaction may, optionally, include cyclic hydrocarbons, such as benzene or cyclohexane, which may undergo ring opening reactions during the course of the isomerization reaction.
The isomerization of paraffins, especially light paraffins, is an established refining process which is traditionally used to provide additional feedstock for alkylation units or to convert relatively low octane linear paraffins to higher octane, branched chain isomers which can be blended into the gasoline pool. Straight chain paraffins such as n-butane, n-pentane and n-hexane are converted to the corresponding isoparaffins by various isomerization processes which may use various types of catalysts.
Non-regenerable Lewis and Bronsted acid catalysts may be used, for example, as disclosed in U.S. Pat. Nos. 3,766,286; 3,852,184; 3,855,346; 3,839,489; 4,144,282; and U.S. Pat. No. 4,814,544. Commercial processes of this type have been developed by various companies including Phillips Petroleum Company (Catalytic Isomerization) and Shell Development Company (Liquid Phase Isomerization).
An alternative type of catalyst used in a number of commercial isomerization processes comprises a metal hydrogenation/dehydrogenation component, usually platinum, on a porous support. An example of this process is the Penex process (UOP) in which the isomerization is carried out in the presence of hydrogen and a platinum catalyst. The Iso-Kel process (M. W. Kellogg) also employs a precious metal catalyst with hydrogen circulation and the Pentafining (Arco/Englehardt) and Butamer (UOP) processes also employ platinum on supports with external hydrogen circulation. Processes of this kind are disclosed, for example, in U.S. Pat. Nos. 4,834,866 (Schmidt) and U.S. Pat. No. 4,783,575 (Schmidt).
Isomerization processes utilizing metal components on supports comprising a molecular sieve are disclosed in U.S. Pat. No. 3,842,114 (Sie); U.S. Pat. No. 3,836,597 (Sie); U.S. Pat. No. 4,778,944 (Zarchy) and U.S. Pat. No. 4,374,296 (Haag).
Paraffin isomerization catalysts may also be employed as ring opening catalysts for the removal of cyclic aromatic precursors from reformer feedstocks as disclosed in U.S. Pat. No. 4,783,575 (Schmidt) and U.S. Pat. No. 4,834,866 (Schmidt). For example, cyclohexane, a precursor of benzene, may be isomerized to a mixture of branched paraffins which are only partly aromatized in the reformer so as to minimize the production of benzene. U.S. Pat. No. 3,631,117 describes a process for the hydroisomerization of cyclic hydrocarbons that uses a zeolite supported Group VIII metal as a catalyst for ring opening and paraffin isomerization. The utilization of paraffin isomerization for ring opening aromatic precursors, especially cyclohexane, is likely to become more important in the future its environmental regulations limit the aromatic content of motor gasoline.